The high-efficiency video coding (HEVC) is a new video compression standard developed by the joint collaborative team (JCT) jointly established by ISO/IEO MPEG and ITU-T. The version 1 HEVC standard for single layer coding was finalized in January 2013, and officially approved by parent body MPEG as MPEG-H and by ITU-T as H.265. The HEVC had a design goal of being at least 50% more efficient than the previous standard MPEG AVC/ITU-T H.264 High profile. To achieve better coding efficiency, HEVC employs a flexible block coding structure. In HEVC, a picture is divided into largest coding units (LCUs) (e.g., up to 64×64 size), and an LCU is further decomposed into coding units (CUs). A CU can be as large as an LCU, and the smallest CU size can be down to 8×8. At the CU level, a CU may be split into prediction units (PUs) with smaller size or equal to CU size for intra- or inter-prediction, and can be split into transform units (TUs) (e.g., with a size from 4×4 to 32×32) for transformation of a prediction residual block.
Within a LCU, some CUs can be intra-coded, while others can be inter-coded. Such a block structure offers coding flexibility of using different PU sizes and TUs sizes based on characteristics of incoming content, especially the ability of using large block size tools (e.g., large prediction unit size up to 64×64 and large transform and quantization size up to 32×32), providing significant coding gain when compared to the 16×16 block based MPEG AVC/ITU-T H.264 coding. After the quantization of transform coefficients, sometimes there are only few isolated small non-zero value coefficients left in a TU. Coding of such isolated small transform coefficients is known to be expensive from the rate-distortion optimization (RDO) point of view.